NOT APPLICABLE
NOT APPLICABLE
FIG. 2 illustrates typical Kohler illumination. The effective source (here a lamp LA) is imaged to the aperture stop AS as LAxe2x80x2. Placing film or other photosensitive material at AS will record the intensity distribution as in J. Kirk, C., Microlithography World, No. 6, 6:25 (1997). However, the aperture stop is not generally accessible for this sort of diagnostic. For a circularly symmetric source, the "sgr" value which is defined as:
"sgr"=NAill/NAo 
where:
"sgr" partial coherence of effective source
NAill numerical aperture of the cone of radiation defining the effective source.
NAo numerical aperture of the aperture stop as seen from the object side (entrance pupil)
xe2x80x83is generally  less than 1. Thus the aperture stop is typically underfilled by the effective source. Control of "sgr" is important for maintaining uniformity of small (xcx9cdiffraction limited resolution) features. Y. Borodovsky, Optical /Laser Microlithography VIII, SPIE 1440:750 (1995) discusses a study wherein "sgr" variation across a stepper FOV resulted in significant linewidth variations. Y. Borodovsky, Optical/Laser Microlithography VIII, SPIE 1440:750 (1995) used micro structures (400 nm spaces at various pitches) and indirectly inferred through image simulations the value of cy. Such an indirect measurement can only capture one or at most a few parameters characterizing the effective source luminous intensity. A direct method of measurement that separates other effects such as imaging objective aberrations, dose control, photoresist development characteristics and provides a more complete set of information is desirable.
Another effect the effective source has on printed imagery arises from decentration of the effective source with respect to the system exit pupil. This also goes by the name of condenser aberrations or alignment. D. Peters, Interface 85, Kodak Publ. No. G-154, 66-72 (1985) describes how condenser alignment leads to printed image distortion that is a function of defocus. It is important to separate this from distortion which is due to the system imaging objective alone. Thus the distortion correcting techniques of McArthur and Smith, U.S. Patent No. 5,929,991; McArthur et al., U.S. Pat. No. 5,392,119; and Macdonald et al., U.S. Pat. No. 5,136,413 would benefit from a metrology tool that could clearly distinguish that part of the distortion due to condenser setup and that part due to the imaging objective alone.
Hasan et al., IEEE Transactions on Electron Devices ED-27, #12 (1980) and Dusa and Nicolau, Optical/Laser Microlithography II, SPIE 1088:354 (1989) utilized electrical methods (van der Pauw resistors) to ascertain condenser alignment. This technique utilized microstructures at different wafer z positions to infer the z dependent distortion described in D. Peters, Interface 85, Kodak Publ. No. G-154, 66-72 (1985). As such, this technique relied on subtracting out the imaging objective contribution to distortion to arrive at condenser misalignments. A measurement technique that intrinsically and clearly separated imaging objective and condenser effects is desirable.
Other techniques aimed at diagnosing imaging objective behavior, not the effective source distribution include: an in situ interferometer for wavefront determination (Smith and McArthur, Apparatus, Method of Measurement, and Method of Data Analysis for Correction of Optical System, U.S. Patent no. 5,828,455 issued Oct. 27, 1998 and Smith and McArthur, Apparatus, Method of Measurement, and Method of Data Analysis for Correction of Optical System, U.S. Patent No. 5,978,085 issued Nov. 2, 1999), K. Freischlad, Optical/Laser Microlithography VIII, SPIE 2440:743 (1995), describes an interferometer (noninsitu) for stepper diagnosis, J. W. Gemmink, Optical/Laser Microlithography II, SPIE 1088:220 (1989) and Brunner et al., Optical/Laser Microlithography VII, SPIE 2197:541 (1994), describe techniques for determining optimal focus; C. Huang, Optical/Laser Microlithography VIII, SPIE 2440:735 (1995), describes techniques for determining optimal focus and astigmatism only; and Dusa and Nicolau, Optical/Laser Microlithography II, SPIE 1088:354 (1989), describes general field characterization and qualification techniques.
The current invention is an insitu device that directly measures the luminous intensity (energy per unit solid angle) of the effective source, it""s alignment, shape, and size. As such it can be readily employed in the devices described by Sato et al., U.S. Pat. No. 4,861,148; T. Whitney, U.S. Pat. No. 5,386,319; K. Jain, U.S. Pat. No. 5,285,236; Yoshitake et al., Optical/Laser Microlithography IV, SPIE 1463:678 (1991); Yudong et al., Optical/Laser Microlithography IV, SPIE 1463:688 (1991); van den Brink et al., Optical/Laser Microlithography IV, SPIE 1463:709 (1991); Unger and DiSessa, Optical/Laser Microlithography IV, SPIE 1463:725 (1991); and Feldman and King, U.S. Pat. No. 3,819,265.
A process of measuring the radiant intensity profile of an effective source of an projection image system that has an effective source, an object plane, an imaging objective, an exit pupil, and an image plane. An array of field points is provided on the object plane of the projection imaging system. A corresponding array of aperture plane apertures is displaced from the object plane a sufficient distance to image the effective source, the array of corresponding object plane apertures corresponding to the field points on the object plane. The improved process consists of selecting at least one field point and a corresponding aperture plane aperture and projecting a plurality of images of the selected field point through the corresponding selected aperture plane aperture at a plurality of various intensities of the effective source. This produces at the image plane a corresponding plurality of images of the effective source image at the selected field point. The effective source images of the selected field points are recorded at the image plane to produce recorded images for each of the plurality of various intensities. By analyzing the recorded images of the effective source it is possible to determine a radiant intensity profile of the image source at the selected field point.